In the past, workers adjusted a (low-pass) filter bandwidth by discretely switching resistor values; e.g. with digitally controlled switches. Alternatively, one could use a digital filter arrangement; however, this method required switchable clock frequencies.
Workers are aware that it would be advantageous to better control the bandwidth of a low-pass filter, especially if this could be done in continuous fashion over several decades of frequency; e.g. a range of 100 Hz to 10 KHz would be very useful.
Thus, a feature of this invention is to provide techniques for controlling a low-pass filter's bandwidth--more particularly to use photoconductive means as variable filter impedance means, and to set resistor values by controlling radiation intensity illuminating the photoconductive elements.
And an added feature is, to provide an associated matched voltage-controlled radiation source. The output radiation intensity of such a source may be controlled according to a variable input control voltage--this control voltage thus acting to set the bandwidth of the filter.
A related feature is to provide such in a circuit array including a "control loop" which compensates for temperature variation of the light source particularly when the control loop comprises a source of regulated voltage and a photoconductor optically linked to the mentioned radiation source so as to be stimulated thereby--to thereby modulate the "control voltage" so as to compensate for temperature variations: Thus, a "temperature-compensating control loop".
In a particular preferred embodiment, a pair of photoconductors are used to set the bandwidth of a two-pole active filter which is responsive to light output from an LED source. The LED source is, in turn, controlled by a control voltage source, CVS. In the CVS, the control voltage is selected to give the prescribed LED output and to consequently adjust the resistance of the photoconductors used in the filter. Preferably, this LED also irradiates a third "reference" photoconductor which is arranged in a feedback control loop to compensate for any temperature-induced output variation in the LED.
Thus, it is an object to address at least some of these concerns and teach ways of ameliorating them. A particular object is to teach the use of photoconductors as variable impedance means in a filter circuit, these being impedance-adjusted by variable illumination means, this, in conjunction with a control loop to compensate for thermal variations. A related object is to do this to yield an analog, voltage-controllable bandwidth, two-pole, active, low-pass filter wherein the bandwidth may be continuously controlled, and varied, over more than two decades of frequency, simply by adjusting a control voltage--using appropriate photoconductors for the filter resistors, and preferably also using a stabilizing photoconductor means in a temperature-compensating feedback loop. A related object is to provide this as part of a continuously-variable, data-aided demodulator.